The present invention relates to cobalt-containing iron oxide pigments, a process for producing the pigments and a magnetic recording medium containing the pigments. More in particular, it relates to cobalt-containing iron oxide pigments having a high coercive force, an excellent S.F.D. (Switching Field Distribution), and small temperature- and time-dependence (an excellent temperature stability and an excellent stability independent on the time) of the coercive force, a process for producing the pigments, and a magnetic recording medium containing the pigments with extremely small print through (extremely small deterioration with passage of time upon transfer) and extremely small long-term erasability (extremely small deterioration with passage of time upon erasure).
Known processes for producing iron oxides having a high coercive force in the prior art comprise essentially a modification of iron oxides with cobalt. The processes can be generally classified into two groups. Namely, a method of mass doping of an iron oxide (so-called doping method) and a method of coating on the surface of the iron oxide (so-called epitaxial method or coating method).
Surface-coating iron oxide pigments are advantageous, for example, in that they are free from deterioration with passage of time upon transfer and deterioration with passage of time upon erasure, excellent in magnetic stability, and of small temperature-dependence and time-dependence of the coercive force. However, in order to obtain a high coercive force, since a large amount of cobalt coated on the surface of iron oxide is required, cobalt costs are high and the resultant coercive force is relatively small as compared with the amount of cobalt used, which is economically disadvantageous.
In DE-A 2,235,383, iron oxide is epitaxially coated with cobalt to obtain a coercive force yield of about 4.5 kA/m.multidot.Co wt %. The coercive force yield is calculated as: coercive force yield=(Hc-27.8)/Co wt %. In this case, 27.8 kA/m as the coercive strength of the analogously Co-free iron oxide is subtracted as compensation. Further, coercive force yields of 9.9, 6.6, 9.5, 5.3 or 6.7 kA/m.multidot.Co wt % are obtained by calculation for analogously epitaxially coated products described in most preferred examples in DE-A 2,629,931, DE-A 2,639,259, DE-A 3,017,525, DE-A 3,017,652 and DE-A 3,101,834. According to other methods as described in DE-A3,631,194 and DE-A 3,843,348, coercive force yields of 9-12 kA/m.Co wt % are obtained.
Generally, the doping method for the iron oxide has a higher coercive force yield. According to the method as disclosed in DE-A 2,252,564, when .gamma.-Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4 is mixed and stirred with an aqueous solution containing cobalt, filtered, washed with water, heated to 250 to 600.degree. C. and then gradually cooled to 100.degree. C. at a rate not more than 10.degree. C./Hr, a coercive force yield as high as 12-16 kA/m.Co wt % is obtained. The effect of the gradual (slow) cooling can also be explained in the context of the distribution of cobalt in a tetrahedral or octahedral site in a spinel lattice (J. Appl. Phys., 1968, Vol. 39, No. 2, page 1204).
Also, a cobalt-doping method which does not use the gradual cooling effect, is also known from DE-A 2,308,791, DE-A 2,413,430 and U.S. Pat. No. 4,224,175. In these cases, a coercive force yields of 12.9, 14.6 and 16.7 kA/m.multidot.Co wt % at the maximum are obtained. In a modern method similar to that described in DE-A 2,903,592, a coercive force yield of about 20 kA/m.Co wt % is attained.
In addition, magnetic iron oxide particles having an excellent magnetic stability such as thermal stability of magnetic properties and comprising cobalt or cobalt and ferric oxide coated on the surface of cobalt-containing iron oxide particles having an excellent thermal stability of magnetic properties, are also known. For instance, Japanese Patent Application Laid-Open (KOKAI) No. 60-93629 proposes magnetic iron oxide particles in which a coating layer composed of a cobalt-containing iron oxide with a cobalt content of not less than 15% by weight based on iron, is formed integrally to the surface of a cobalt-containing iron oxide (.gamma.-Fe.sub.2 O.sub.3) crystal with a cobalt content of not more than 5% by weight based on iron [for example, cobalt content (Co/Co-.gamma.-Fe.sub.2 O.sub.3) in examples are 2.464% by weight and 3.64% by weight]. Japanese Patent Application Laid-Open (KOKAI) No. 55-72009 proposes a process for producing magnetic iron oxide particles, which comprises mixing cobalt-containing iron oxide particles with a cobalt salt in an alkali solution, heating the resultant mixture, and depositing a cobalt compound on the surface of the cobalt-containing iron oxide particles.
When cobalt is uniformly doped to a magnetic iron oxide particle, a high coercive force and an excellent S.F.D. can be attained easily. The tendency that the coercive force becomes higher is more conspicuous as the amount of the cobalt is increased. Therefore, the cobalt-containing iron oxide pigment prepared by the Co-doping method is obtainable at a lower cost than that obtained by the Co-coating method. However, as the amount of cobalt increases, it result in a drawback that the magnetic stability is worsened (demagnetization under pressurization and demagnetization under heating arise), the print through (deterioration with passage of time upon transfer) and long-term erasability (deterioration with passage of time upon erasure) arise, and the temperature stability and the stability independent on the time of coercive force is deteriorated.
When cobalt or cobalt and ferric oxide is coated on the surface of a magnetic iron oxide particle, although the print through (deterioration with passage of time upon transfer) and long-term erasability (deterioration with passage of time upon erasure) do not arise, and the temperature stability and stability independent on the time of the coercive force are excellent, it becomes difficult to obtain a high coercive force or an excellent S.F.D.
As a result of the present inventors' studies, it has been found that by integrally forming as a surface layer, a coating layer composed of a cobalt-containing iron oxide having a specific average Co concentration, for instance, at least 5 times as high as the average Co concentration of a berthollide core particle, on the surface of the berthollide core particle in which an extremely small average Co concentration, for instance, 0.1 to 1.0% by weight (calculated as Co) based on an iron oxide in the core particle is distributed evenly and in which the Fe.sup.2+ content is from 16 to 22% by weight based on the iron oxide contained in the core particle, the obtained cobalt-containing iron oxide pigments have a high coercive force, an excellent S.F.D., and small temperature- and time-dependence (an excellent temperature stability and an excellent stability independent on the time) of the coercive force. On the basis of the finding, the present invention has been attained.